Photographic materials having a silver halide emulsion layer are generally subject to various mechanical stresses. For example, a negative film for general photographing is bent on putting into a cartridge(patrone) in roll or loading into a camera or pulled on film winding.
Various stresses imposed on a light-sensitive material are transmitted to silver halide grains via gelatin as a binder for silver halide grains or a plastic film support. It is known that a stress imposed on silver halide grains induces a change in photographic performance of the light-sensitive material, as reported, e.g., in K. B. Mather, J. Opt. Soc. Am., Vol. 38, p. 1054 (1948), P. Faelens and P. de Smet, Sci. et Ind. Phot., Vol. 25, No. 5, p. 178 (1954), and P. Faelens, J. Photo. Sci.,Vol. 2, p. 103 (1954).
Various means have been studied to reduce the change in photographic performance due to stressing (hereinafter referred to as stress sensitiveness). For example, a means for reducing stress sensitiveness by making a difference in iodide content in the inside of individual grains, so-called an iodine gap, is disclosed in JP-A-59-99433, JP-A-60-35726, and JP-A-60-147727 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Further, a technique for reducing stress sensitiveness while assuring satisfactory photographic sensitivity by making an iodine gap not only in the inside thereof but between a surface layer and an inside layer is disclosed in JP-A-62-123445. These means surely reduce change of photographic performance caused by the imposed stress but are accompanied by deterioration in latent image preservability and incubation resistance and are therefore unsatisfactory.
On the other hand, the idea of controlling the iodide distribution within individual-silver halide grains like the above-mentioned techniques has also been studied for the purpose of improving photographic characteristics other than reduced stress sensitiveness. For example, JP-A-58-113927 discloses a means for improving sensitivity, graininess and sharpness by providing a high iodide layer in the inside of grains. However, the silver halide grains according to this technique undergo significant change of fog during storage. JP-A-58-113927 discloses a tabular silver halide emulsion having an improved sensitivity to granularity ratio by increasing the iodide content on the outermost layer. However, the emulsion has turned out to have too poor latent image preservability to withstand practical use. JP-A-62-58237 discloses a means for improving preservability comprising providing an iodide conversion layer in the inside of individual grains thereby to prevent change of fog with time. This technique was still insufficient for improving latent image preservability.
While a variety of techniques have thus been attempted, they are to improve or enhance a part of the photographic performance but never to satisfy all the properties, such as reduced stress sensitiveness, sensitivity to granularity ratio, latent image preservability, incubation resistance, and so on.